1. Field of the Invention
The invention relates to a method and system for wireless communications, more particularly to a method and system for real-time identification of occurrence of shadowing phenomenon during wireless communications.
2. Description of the Related Art
In recent years, due to the conveniences of mobility and less restrictions on communications locations offered by wireless communications, people have become accustomed to using wireless communications to communicate with others. For example, people own at least one mobile phone. In addition, wireless network cards have become a basic component of new consumer electronic products.
When a wireless signal transmission path is blocked due to interference by an obstacle (such as a building) there will be a substantial loss in signal power. This phenomenon is known as fading, which is considered to be a very important phenomenon in the field of wireless communications. There are two main types of fading, i.e., fast fading and long-time fading. Long-time fading is also known as shadowing. Many schemes, such as error control coding and re-transmission, have been proposed heretofore to mitigate the effects of fast fading. In some voice channel applications, the effects of fast fading are actually ignored because they usually do not affect communications content among humans. However, shadowing cannot be ignored because it may last for a considerable amount of time and may result in abrupt failure in communications.
In order to ensure the quality of wireless communications, a signal-receiving device of user end equipment (such as a mobile phone) or system service end equipment (such as a base station) will periodically execute a synchronization process to generate a set of channel-transmission-state estimated values that includes power and phase parameters, such as those defined in Section 5.1 of ETSI TS 125 211 V6.4.0 (March, 2005), for correcting the effects of wireless signal transmission paths on transmitted signals. At present, since actual changes in phase parameters during real world communications are complex and difficult to analyze, majority of current communications system service end equipments refer solely to the power parameters as basis for evaluating quality of wireless communications and for determining whether handoff between base stations should be conducted. For instance, mobile phones will periodically transmit Keep_Alive signals to base stations, and base stations respond to the Keep_Alive signals by sending ACKnowledge signals to mobile phones. However, when the signal transmission path between a base station and a mobile phone is blocked, the shadowing phenomenon occurs, and the mobile phone that was unable to receive the ACKnowledge signal will automatically increase the transmission power for the Keep_alive signal to a maximum and adjust the frequency of transmissions of the Keep_Alive signal to a maximum, thereby resulting in large consumption of power resources of the mobile phone. In the case that the signal transmission path experiences complete shadowing phenomenon, such as when the mobile phone is inside an elevator, the above tasks performed by the mobile phone result in wasteful power consumption, and do not lead to an improvement in communications quality.